A lot of people talked about having to deal with group members in greens, generally attributing it to the lowering of the reputation needed to gain a heroic key. I have to say that I fully support lowering the heroic key requirement to honored, if only for the sake of my alts, and I know that in many cases at the 5-man level, skill is greater than gear. However, when a prospective tank shows up in level 62 greens, there might be a problem. No matter how skilled a tank is at keeping aggro, if they only have 8k health, it's going to be a hell of a time keeping them up. Plus, the skilled tanks generally pride themselves on keeping their gear up to date anyway, at least most of the time.
Then there was the talk about people being asked to do stuff that wasn't really in their power, like the fury warrior being asked to sap. Yeah, sometimes it's hard to know how some aspects of the class work if you haven't played it, and I'll admit that I have asked a Mage to Polymorph an elemental (and felt dumb about it straight afterwards), but there's some point at which you should know the very basics about what each class can do so you can work together with them, right?
Of course, a few other points people made I was a bit more lenient on. Some people say they leave if the tank doesn't have threat meters. I'm crazy for Omen myself, and I don't play without it, but I don't expect everyone to have the same addons I do when it comes to a casual PuG. Maybe it's because I played Everquest (Which never had threat meters to the best of my knowledge) for so many years, but I don't mind playing with someone without a threat meter and just being careful with my damage on a 5-man level. That said, If I'm not tanking on my Druid, I'm probably playing my Hunter, and then I can just feign off the threat. For a class without an easy threat dump, I'm sure having threat meters is a lot more needful.
Oh, if I were to add my own example of how I knew my group was in trouble, I'd add one really lovely PuG I had in Sethekk Halls where the mage would actually attack me every time I got mind controlled, once succeeding in killing me and wiping the group. Her explanation? "I couldn't help it, it was too tempting!"
Of course, we DID finish that run -- after a few more wipes.
What about your own horror stories? Have you had the Heroic PuG from hell? Where do you draw the line and leave? Do greens make you grimace? Do you insist that everyone use certain addons? Or is it more of a skill or knowledge issue?